Development and first implementation of a novel multi-modality cardiac motion and dosimetry phantom for radiotherapy applications

Abstract

Background: Cardiac applications in radiation therapy are rapidly expanding including magnetic resonance guided radiation therapy (MRgRT) for real-time gating for targeting and avoidance near the heart or treating ventricular tachycardia (VT).

Purpose: This work describes the development and implementation of a novel multi-modality and magnetic resonance (MR)-compatible cardiac phantom.

Methods: The patient-informed 3D model was derived from manual contouring of a contrast-enhanced Coronary Computed Tomography Angiography scan, exported as a Stereolithography model, then post-processed to simulate female heart with an average volume. The model was 3D-printed using Elastic50A to provide MR contrast to water background. Two rigid acrylic modules containing cardiac structures were designed and assembled, retrofitting to an MR-safe programmable motor to supply cardiac and respiratory motion in superior-inferior directions. One module contained a cavity for an ion chamber (IC), and the other was equipped with multiple interchangeable cavities for plastic scintillation detectors (PSDs). Images were acquired on a 0.35 T MR-linac for validation of phantom geometry, motion, and simulated online treatment planning and delivery. Three motion profiles were prescribed: patient-derived cardiac (sine waveform, 4.3 mm peak-to-peak, 60 beats/min), respiratory (cos⁴ waveform, 30 mm peak-to-peak, 12 breaths/min), and a superposition of cardiac (sine waveform, 4 mm peak-to-peak, 70 beats/min) and respiratory (cos⁴ waveform, 24 mm peak-to-peak, 12 breaths/min). The amplitude of the motion profiles was evaluated from sagittal cine images at eight frames/s with a resolution of 2.4 mm × 2.4 mm. Gated dosimetry experiments were performed using the two module configurations for calculating dose relative to stationary. A CT-based VT treatment plan was delivered twice under cone-beam CT guidance and cumulative stationary doses to multi-point PSDs were evaluated.

Results: No artifacts were observed on any images acquired during phantom operation. Phantom excursions measured 49.3 ± 25.8%/66.9 ± 14.0%, 97.0 ± 2.2%/96.4 ± 1.7%, and 90.4 ± 4.8%/89.3 ± 3.5% of prescription for cardiac, respiratory, and cardio-respiratory motion profiles for the 2-chamber (PSD) and 12-substructure (IC) phantom modules respectively. In the gated experiments, the cumulative dose was <2% from expected using the IC module. Real-time dose measured for the PSDs at 10 Hz acquisition rate demonstrated the ability to detect the dosimetric consequences of cardiac, respiratory, and cardio-respiratory motion when sampling of different locations during a single delivery, and the stability of our phantom dosimetric results over repeated cycles for the high dose and high gradient regions. For the VT delivery, high dose PSD was <1% from expected (5-6 cGy deviation of 5.9 Gy/fraction) and high gradient/low dose regions had deviations <3.6% (6.3 cGy less than expected 1.73 Gy/fraction).

Conclusions: A novel multi-modality modular heart phantom was designed, constructed, and used for gated radiotherapy experiments on a 0.35 T MR-linac. Our phantom was capable of mimicking cardiac, cardio-respiratory, and respiratory motion while performing dosimetric evaluations of gated procedures using IC and PSD configurations. Time-resolved PSDs with small sensitive volumes appear promising for low-amplitude/high-frequency motion and multi-point data acquisition for advanced dosimetric capabilities. Illustrating VT planning and delivery further expands our phantom to address the unmet needs of cardiac applications in radiotherapy.

Keywords: MRgRT; cardiac phantom; dosimetry; multi‐modality.

Figure 1.

Patient-informed heart model contoured on a contrast-enhanced CT coronary angiography (CCTA) scan (left), exported as a 3D model and post-processed (center), then 3D-printed on a FormLabs 3BL printer using Elastic50A (right).

Figure 2.

(A) Two custom phantom module inserts assembled with dosimeters containing a whole heart derived from a cardiac substructure model and A1SLMR (left) and two-chamber model to embed HS-RP200 multi-point plastic scintillators (right). (B) Example whole heart A1SLMR module placed in a custom external housing and attached to a commercially available MR-safe motor for use in MRI and MR-linac settings. The MR-compatible phantom was imaged on a 0.35 T MR-linac with blue torso posterior receive coils shown.

Figure 3.

Waveforms imported into the phantom software for cardiac phantom motion verification and dosimetry experiments. The top row indicates the prescribed motion while the bottom row reflects the measured motion from analyzed cine data with programmed maxima and minima shown for the 2-chamber phantom module that contains the plastic scintillation detectors. (Top left) Respiration was represented with a cos⁴ curve (12 breaths per minute, 3.0 cm peak-to-peak). (Top middle) Combined respiratory and cardiac motion was represented with cos⁴ and sine curves (12 breaths per minute/70 beats per minute, 24 mm/4 mm peak-to-peak for respiratory and cardiac waveforms, respectively). (Top right) Isolated cardiac motion derived from a cardiac MRI sequence (62 beats per minute, 4.3 mm peak-to-peak). Ranges of prescribed motion are shown by dashed lines.

Figure 4.

Multi-target plan using the 2-chamber module with four Plastic Scintillator Detector (PSD) active volumes contoured and numbered. The Right Ventricle (RV), Left Ventricle (LV), and PSD channels were visualized and contoured as regions of interest.

Figure 5.

Cardiac phantom x-ray-based setup images, with simulation CT (SIM CT) and cone-beam CT (CBCT) fusion checkerboard (top), and dose distribution for a ventricular tachycardiac treatment plan (bottom) shown. Plastic scintillation detectors 1–4 along with PTV (red) and GTV (purple) contours are shown on axial (top left) and coronal (top right) images. Dose distribution ranging from 6.00 to 28.75 Gy (prescribed dose = 25.00 Gy) are shown for axial (bottom left) and coronal (bottom right) slices. PTV, planning target volume; GTV, gross tumor volume.

Figure 6.

(a) Axial slice of patient contrast-enhanced coronary computed tomography angiography (CCTA) scan with contours used to generate the 3D-printed heart model. (b) Corresponding axial low-field TrueFISP MR image of derived cardiac phantom (12 substructures) with an A1SL MR ion chamber cavity. (c) Axial MR image of two-chamber (left and right ventricle) cardiac model with four plastic scintillation detector channels highlighted. MR, magnetic resonance.

Figure 7.

Real-time plastic scintillator detector (PSD) readouts for ungated cardiac, respiratory, and cardio-respiratory waveforms acquired with an acquisition rate of 10 Hz for each detector. Note the sensitivity of the measurement for the rapidly fluctuating cardiac motion and the varied readout based on PSD location and the associated motion profile.